More than 150 scientists, surgeons and doctors have written to NHS professional bodies calling on the medical establishment to demand that the government withdraws its controversial health bill.

Co-ordinated by the NHS Consultants' Association, the medics have written to presidents of the royal medical colleges urging them to stop co-operating with the government's proposed NHS reforms.

The move comes as the British Medical Association begins to mobilise a public campaign against the bill, and coincides with the suggestion of Clare Gerada, chair of the Royal College of General Practitioners, that family doctors hire lawyers to cope with the conflicts of interest they would face over the commissioning reforms.

The letter says the health bill, devised by the health secretary, Andrew Lansley, is not supported by the majority of the medical profession and is not in the best long-term interests of either patients, doctors or the royal colleges.

The plan would, the doctors argue, lead to "marketisation and privatisation" of the English NHS, as well as promote competition with a new regulator, and remove the health secretary's duty to provide a comprehensive health service. The letter highlights a poll of more than 1000 doctors from the British Medical Journal, showing 93% want Lansley's bill withdrawn, and suggests there is a lack of democratic legitimacy. The government needs to "reform its reforms", following the public and professional backlash this year, and the changes have been expensive, the writers say: savings from the changes would bring in £4.5bn over the next four years, £700m less than the government first envisaged.

The doctors are also concerned at the emollient tone of some royal colleges. The Academy of Medical Royal Colleges told MPs there were too "many disadvantages" in delay, while Norman Williams, president of the Royal College of Surgeons, said his body largely backed "the aims of the reforms to modernise the health care system". The letter claims "colleges are out of touch with the views … of the majority of grassroots doctors", and accuses them of failing to safeguard their own principles, a key role being to "promote the underlying principles of medical professionalism and leadership".

The bill, the letter says, cannot pass without the medical profession's support.

"The colleges have a rare opportunity to make a stand for the NHS, medical profession, and patients. We therefore call upon the Academy of Medical Royal Colleges to act in the public interest by publicly calling for the withdrawal of the health and social care bill."

Clive Peedell, a consultant oncologist and BMA council member, who worked behind the scenes to circulate the letter, said that though the bill had passed through parliament there was a groundswell of medical opinion against it. "We are saying that the bill may have been passed by MPs but doctors are against these changes and we have to mobilise to stop it."

Peedell publicly praised the Liberal Democrats who voted against the bill, saying medics would be backing grassroots efforts to get the bill debated at the party's annual conference.

Evan Harris, the former Liberal Democrat MP, who has become the party's most influential anti-NHS bill campaigner, noted that two-thirds of English Lib Dem backbenchers do not back the government – one of the biggest rebellions of Nick Clegg's leadership.

Among those who did not back the reforms were the party president, Tim Farron, the deputy leader, Simon Hughes, and John Pugh, chair of the Lib Dem backbench committee on health.

The bill now goes to the Lords, where Baroness Williams is set to play a leading role, having voiced concerns over the influence of the private sector and warned of the battle being "far from over".